1. Field of the Invention
This invention relates to a vibration wave driven motor in which a movable member is frictionally driven by a travelling vibration wave, and to a cooling structure for cooling a heat generating portion.
2. Related Background Art
A proposal relating to the structure of a vibration wave driven motor utilizing the bending vibration of a piezo-electric member has been made by Japanese Laid-Open Patent Application No. 63-73887.
This proposal provides:
1) a compact vibration wave driven motor; PA1 2) a vibration wave driven motor excellent in heat radiating property; PA1 3) a vibration wave driven motor of high efficiency; and PA1 4) a vibration wave driven motor in which pressure force regulation is easy.
The principle of driving of a vibration wave driven motor is known and therefore will hereinafter be described not in detail, but briefly.
In a vibration wave driven motor, an electro-mechanical energy conversion element, for example, a piezo-electric element is adhesively secured to one side of a resilient member made of a metal and formed, for example, into a circular ring shape, and AC voltages differing in phase from each other are applied to two groups of driving piezo-electric elements formed on said piezo-electric element to thereby excite two standing waves on the resilient member, and a travelling vibration wave which is a bending vibration is formed by the combination of these standing waves.
On the other hand, for example, a member of circular ring shape is pressed against the other side of the resilient member with pressing means such as a spring interposed therebetween, and this member or the resilient member is moved by frictional driving provided by a travelling vibration wave formed in the resilient member.
An example of the prior art will hereinafter be described with reference to FIG. 3 of the accompanying drawings.
The reference numeral 2 designates the cover of a motor, and the reference numeral 1 denotes the case of the motor. A piezo-electric member 4 is secured to a resilient member 3 of circular ring shape, and these constitute a stator (a vibration member) 5.
The motor is of such structure that the stator 5 generates heat by heat energy created by strain caused by the supply of electric power to the piezo-electric member 4 and the temperature rise of the motor by the heat generation of the stator is conducted from the stator 5 to the cover 2 and the case and is radiated thereby.
The reference numeral 8 designates a rotor constructed by a slider 7 being secured to a ring 6. This rotor 8 is adapted to be urged against the stator 5 by the pressure force of a countersunk spring 10 with a rubber member 9 interposed therebetween, and to be rotated with a shaft 11. The pressure force can be very easily regulated because the thickness of a shim 12 is suitably chosen and regulated and then the shim is held by a snap ring 13. The shaft 11 is rotatably supported by bearings 14 and 15 mounted in the case 1 and the cover 2, respectively.
Now, in the above-described example of the prior art, the transmission routes of the heat energy created in the stator 5 during driving include three routes, i.e., the heat radiation from the surface of the stator 5, the route from the driving surface to the rotor, and the route from the thin-walled portion 3-a of the stator through a stator mounting portion- 3-b to the cover 2 and the case 1. Of these, the radiated heat from the surface of the stator 5 and the heat energy conducted to the rotor 8 are very slight. Thus, most of the heat energy is conducted and radiated through the third route, i.e., the route from the thin-walled portion 3-a of the stator via the mounting portion 3-b to the cover 2 and the case 1.
In this case, the quantity of heat conducted per unit time is proportional to the cross-sectional area perpendicular to the direction of conduction. Therefore, in the example of the prior art, the cross-sectional area of the thin-walled portion for endowing the stator with flexibility is very small and the quantity of heat which can be conducted and radiated to the outside of the motor per unit time is very small. This has resulted in the overheating of the stator 5 which in turn has sometimes led to the deterioration of the motor characteristic by the temperature dependency of the stator resonance frequency, or to the reduced strength or peeling-off of the adhesive which secures the piezo-electric member 4 to the resilient member 3.